Cover-plug for the core of a fast neutron nuclear reactor

Abstract

Cover-plug for the core of a fast neutron nuclear reactor, comprising a support-plate (6), a cylindrical shell (8) having a vertical axis fixed integrally to the support plate (6) and braces (14) which are transverse relative to the shell (8). Vertical tubes fixed to the support plate (6) at their upper part are joined to the braces (14), which consist of at least two conical webs (14) having their axes coincident with the axis of the shell (8). These conical webs have a top angle greater than 120° and a diameter which is slightly smaller than the diameter of the shell (8), at their base. The brace (14a) situated in the lowest position in the shell (8) ensures the deflection of the reactor cooling fluid. The invention applies, in particular, to fast neutron nuclear reactors of the integrated type.

I claim: 
     
       1. A fast neutron nuclear reactor comprising a reactor vessel, a core, control rods, a cooling fluid contained in said reactor vessel, core instrumentation and a cover plug comprising a support plate resting on a turning plug (1) closing said reactor vessel, around an opening for passage (7) of said core cover-plug (4) into said turning plug (1), a cylindrical shell (8) having a vertical axis and fixed integrally to said support plate (6) at its upper part, a plug for radiation protection (11) occupying the internal volume of said cylindrical shell over a height which corresponds substantially to the thickness of said turning plug, and a set of vertical tubes (10) for receiving said control rods, said vertical tubes being fixed to said support plate (6) at their upper ends, arranged within said cylindrical shell (8) and connected to braces (14) having outer edges and arranged transversely relative to said shell (8), wherein (a) a set of said braces (14) which are fixed to said tubes (10), inside said shell (8) consists of at least two vertically spaced conical webs (14) having an axis coinciding with the axis of said shell (8), a top angle greater than 120° and an external diameter which is slightly smaller than the internal diameter of said shell (8), a clearance existing between the inner surface of said shell (8) and said outer edge of said braces (14);   (b) said braces (14) are held in an axial position relative to said cylindrical shell (8) by abutments (15) which are integral with said shell (8) each of said braces having openings only large enough to allow passage of said vertical tubes therethrough, the upper of said at least two braces having additional passages allowing passage of sodium therethrough; and   (c) the lowermost brace (14a) is free of any additional sodium passage holes and is arranged with its outer surface directed downwardly, said lowermost brace being held by said abutments just above the lower part of said shell (8) said shell being pierced with openings (18, 19) only in the lower portion thereof, said openings (18, 19) being on either side of said lowermost brace (14a) enabling deflection of said cooling fluid.   
     
     
       2. A fast neutron nuclear reactor as claimed in claim 1, wherein said vertical tubes (10) are continuous over their entire length and are closed in their upper part, in the region of said support plate (6). 
     
     
       3. A fast neutron nuclear reactor as claimed in claim 1, in the case of a cover plug having a diameter slightly under five meters, which comprises at least two said braces consisting of conical shells having a top angle close to 160°, arranged within said cylindrical shell with a clearance close to 30 mm. 
     
     
       4. A fast neutron nuclear reactor as claimed in claim 1, comprising in its lower part a flat horizontal grid, fixed to said tubes (10) and comprising openings for the passage of instrumentation tubes (20) as well as holes for passage (27) of cooling fluid, said grid (23) consisting of several identical parts placed side-by-side, fixed to said tubes (10) and held in relation to each other by means of said tubes (10). 
     
     
       5. A fast neutron nuclear reactor as claimed in claim 4, wherein said instrumentation tubes (20) are fixed to said lowermost brace (14a) in the region of openings provided in the latter and are fitted in holes (24) of said grid. 
     
     
       6. A fast neutron nuclear reactor as claimed in claim 4, wherein said instrumentation tubes (20) are fixed to a second brace (14b) arranged just above said lowermost brace (14a) for deflecting said cooling fluid through openings provided in said second brace (14b) and are fitted with a small clearance in the openings provided in said lowermost brace (14a).